1. Field of the Invention
This invention relates to the field of synthetic cables. More specifically, the invention comprises a cable termination which allows a cable to freely flex without placing excessive stress on the cable strands.
2. Description of the Related Art
Devices for mounting a termination on the end of a cable are disclosed in detail in copending U.S. application Ser. No. 60/404,973 to Campbell, which is incorporated herein by reference.
The individual components of a wire rope are generally referred to as “strands,” whereas the individual components of synthetic cables are generally referred to as “fibers.” For purposes of this application, the term “strands” will be used generically to refer to both.
Some type of fitting must typically be added to a cable in order to transmit a load to the cable. An old example of this idea is to wrap one end of a cable back upon itself—usually around an “eye” or “thimble” device—then clamp the cable to itself with one or more U-bolts. The resulting assembly on the end of the cable is referred to as a “termination.”
It is known to terminate the strands of a synthetic cable by locking them into an anchor. The strands can be locked in place using a mechanical clamp, solidified potting compound, or other known approaches. The use of potting compound is perhaps the most common. For this approach, the strands are typically splayed into a diverging pattern and infused with liquid potting compound (using a variety of known techniques). The liquid potting compound is any substance which transitions from a liquid to a solid over time. The most common example would be a cross-linking adhesive such as an epoxy. Those skilled in the art know that such adhesives use two separate liquids which cross-link when mixed together. Such a liquid is mixed just prior to wetting the strands.
The wetted strands are at some point placed in a cavity within the anchor (in some cases prior to wetting and in some cases after wetting), so that when the liquid potting compound hardens the strands will be locked to the anchor. The anchor and the portion of cable locked therein are then collectively referred to as a termination.
FIG. 1 shows a prior art termination 14 for a synthetic cable (in a sectional view). Anchor 18 features an expanding cavity 28 joined to a straight portion 38. The hardened potting compound forms potted region 16, in which the strands are locked rigidly in place. The portion of cable 10 below the anchor (with respect to the orientation shown in the particular view) is relatively free to flex. The transition from the freely flexing portion of the cable to the portion locked within the potting compound is denoted as potting transition 20.
The reader should at this point consider the differences between traditional wire rope strands and modern synthetic cable strands. Wire rope strands are relatively large, relatively stiff, and have a moderate surface coefficient of friction. Synthetic cable strands are, in comparison, quite small, have very little stiffness, and have a very low coefficient of friction. Synthetic strands are analogous to human hair in terms of size and stiffness. These differences mean that termination techniques traditionally used for wire rope cannot be used for synthetic cables—or at least not without substantial modification.
Those skilled in the art will know that the maximum theoretical stress a cable can withstand (force per unit area) is 100% of the maximum theoretical stress an individual strand can withstand. In practice, of course, the cable as a whole never reaches 100% of the strand strength. In wire rope applications, an ultimate cable stress of 70% of the individual strand stress is quite good.
Of course, numerous other factors degrade the ultimate stress a cable can withstand. Bending of the cable is perhaps the most significant of these. A cable is ideally loaded while in perfect alignment. Deviations from this alignment degrade the performance. One particularly worrisome situation is where a cable is fixed at one end within an anchor and the freely flexing portion is then bent with respect to the anchor. FIG. 9 shows such a situation.
Wire ropes tolerate this condition fairly well. Their strand stiffness—the strands are typically steel—preserves the cable's circular cross section as it passes through an arcuate bend. The stiffness—as well as the internal friction between the strands—means that the strands stay well organized. Thus, the loss of ultimate tensile strength a wire rope experiences when undergoing a bend is manageable.
This is not true for synthetic cables. FIG. 2 shows a synthetic cable termination undergoing a significant bend. Flexible region 30 of cable 10 has been pulled to one side, forming a first kink 22 where the cable exits the anchor and a second kink 72 where the cable exits the potted region. These two kinks—which may be significantly different in nature—place considerable stress on the individual strands, and may even break or cut some strands. The cable has also flattened substantially in the region of second kink 72. The result is that the majority of the load is carried by a relatively small number of strands.
FIG. 3 shows another type of prior art anchor 18. The version shown does not include a straight portion. A relatively sharp corner is present proximate potting transition 20. This sharp corner exacerbates the problem seen in FIG. 2, since the sharp corner may actually cut synthetic strands which are forced against it (Solidified potting compound often creates a very sharp edge).
FIG. 3A shows a greatly magnified view of potting transition 20. The portion of the strand 32 lying within potted region 16 is held in alignment. Where it exits the hardened potting compound, however, it undergoes an immediate sharp bend. This bend produces stress concentration 66. FIG. 3A represents a very uniform (“good”) potting transition. However, the reader will perceive how substantial stress concentration in individual strands can nevertheless occur. FIG. 4 shows the kinking of the individual fibers against a sharp corner where they exit an anchor. Strands at this point are subject to axial compression and bending compression. Such lateral loading are often cyclic in nature, resulting in “flex fatigue” (a condition of accumulating plastic deformation or outright breakage of the individual cable strands).
The strands actually forced against the corner may even be cut. Synthetic cable strands have little cut resistance in comparison to wire rope strands. This fact represents yet another difference between synthetic cables and wire ropes. Strand cutting is a much larger concern for synthetic cables.
Looking now at FIG. 5, the reader will note that the potting transition 20 is typically irregular in shape, since the infusion of the liquid potting compound through the strands may not be uniformly planar. A portion of hardened potting compound can extend into the freely flexing region of cable near the cable's centerline. This portion often breaks free when the cable is flexed laterally. The existence of the solid region—even when broken free—tends to kink and abrade the cable's strands. Some prior art anchors have included features which could mitigate the aforementioned problems somewhat (at least insofar as they reduce an edge actually cutting into the cable). These features are typically the result of manufacturing convenience or cosmetics, rather than any specific attempt to address the problem of flexural loads. FIG. 6 shows an anchor 18 having a small fillet 24 around its lower edge (the fillet joins the lower surface and, in this case, the wall of straight portion 38). (Throughout this disclosure, directional terms such as “upper” and “lower” will be understood to refer only to the orientation shown in the view. The devices disclosed will obviously function in any orientation).
FIG. 7 shows an anchor 18 having a small chamfer 26 around its lower edge. Such a chamfer is sometimes added to prevent a sharp corner existing at the bottom of expanding cavity 28 (For an anchor having no straight portion, this feature can be particularly important). Such fillets and chamfers have traditionally been added to facilitate machining of the anchors on a lathe or automatic screw machine. Those skilled in the art will know that a sharp corner at the mouth of a bore is undesirable for such machining.
While some flex-mitigating features are found in the prior art terminations, they do not readily accommodate substantial lateral flexing of the cable. Thus, when such terminations are attached to an object, the attachment must allow the anchor to move freely so that it remains aligned with the cable. Suitable attachments include ball and socket joints. However, it is often desirable to attach the anchor to an object without allowing any movement. An example would be an externally threaded anchor which is threaded into a hole in a plate. Once installed, the anchor will be rigidly held.
The prior art includes certain strain-relieving devices. FIG. 16 shows the addition of a soft boot 44 encircling the portion of cable 10 which is immediately adjacent to anchor 18. Made of a pliable material—such as a hard rubber—the soft boot can reduce strand kinking. FIG. 17 shows another type of boot—designated as external boot 46. This version attaches to the outside surface of anchor 18, while still surrounding the portion of the cable which is adjacent to the anchor.
Unfortunately, it is difficult to design a soft boot which can accommodate the different loads and different bending angles which can be placed on a cable. FIG. 18 shows a soft boot using a relatively stiff material. The cable tends to bend near the exit of the boot, causing bend point 48. Thus, the unwanted bend has merely been shifted downward rather than eliminated.
In order to reduce this phenomenon, the designer will often substitute a more pliable compound. Such a pliable compound has been used in FIG. 19. However, at higher loads or angles, a bend point 48 still results, albeit in a higher location. The reader will thereby appreciate the difficulty in optimizing the boot stiffness using the prior art approach. Thus, while the prior art devices can reduce problems associated with the lateral flexing of a cable, a more advanced solution is desirable.